Image sensors are devices that can transform optical images into electrical signals. Image sensors are typically classified into charge-coupled devices (CCD) and CMOS image sensors. The CCD has a plurality of MOS capacitors and operates by moving charges that are generated by optical light. The CMOS image sensor includes a plurality of unit pixels and a CMOS circuit controlling output signals from each unit pixel.
The CCD has several disadvantages such as requiring relatively complicated operation and manufacturing processes, consuming relatively large amounts of power, and being difficult in integrating a signal processing circuit on a CCD chip. A CMOS image sensor, however, can be more easily fabricated because CMOS image sensors can be manufactured using conventional CMOS technology.
Conventional CMOS image sensors may be degraded because of reductions in charge transmission efficiency and reductions in charge storage capacity due to noise or dark currents. Dark currents, which result from the accumulation of charges without optical incidence from photo-detecting devices, have been treated as being generated from silicon dangling bonds or defects on silicon substrate surfaces; however, hot carriers may also be a major factor in generating dark currents.
As described in an article by C. C. Wang et al., entitled “The Effect of Hot Carriers on the Operation of CMOS Active Pixel Sensors,” IEDM Tech. Dig., 2001, pp. 563-566, hot carriers arising from transistors within the active pixel sensor, specifically, from pinch-off regions of source follower transistors, can raise a substrate potential and make drain-to-source currents (Ids) of the transistors higher. As these currents are increased, the hot carriers are further generated to thereby increase the dark current and degrade image quality effect.